How many criteria are needed to sustain a reentry circuit?

• A. Two Criteria
• B. Three Criteria
• C. Four Criteria
• D. One Criterion

Answer: (B)

A self-sustaining reentry circuit needs three ingredients: a path that forms a circuit around an obstacle, a unidirectional block in part of that circuit so the impulse can travel in only one direction around the loop, and a conduction time around the loop that is long enough so the tissue has recovered excitability by the time the wavefront returns to it. If any of these fails, reentry cannot be sustained: without a circuit there’s nowhere to loop; without a unidirectional block the impulse travels in both directions and doesn’t create a circulating wavefront; without slow enough conduction (or the right timing relative to refractory periods) the tissue ahead is still refractory when the wavefront comes back, so it cannot re-excite and the loop terminates. That’s why three criteria are required.